Interfacial tension between two immiscible fluids can be controlled by electrical potential. This basic principle is used in a diverse and continuously growing group of electro-optical modulated displays that include devices in which the optical state of an imaging material is modulated or changed by subjecting the imaging material to an electric field or the transport of electrons, for example, electrowetting, electrophoretic or electrochromic devices.
Electrowetting has become an attractive modulation scheme for a variety of optical applications due in part to a desirable combination of high brightness and contrast ratio, a large viewing angle, and a fast switching speed. In addition, the power consumption of electrowetting displays is relatively low because they do not require front or backlighting. For example, electrowetting has been used to provide optical switches for fiber optics, optical shutters or filters for cameras and guidance systems, optical pickup devices, optical waveguide materials, and video display pixels. The term “electrowetting” describes the effects of an electric field on the contact angle of a liquid with a hydrophobic surface. With an electric field, the liquid distributes over, or wets, a surface that initially repels the liquid resulting in a change in the spectral properties of a device. When the electric field is removed, the contact angle increases and the liquid contracts into an area whereby the spectral properties are returned to the initial state.
Early electrowetting systems employed water and mixtures of water and other polar solvents. However, the physical properties of water, such as expansion at higher temperature and freezing point, limit the applications for such devices. To overcome problems associated with the use of water, other solvents, such as glycols, propylene carbonate, diethylcarbonate, diacetone alcohol, cyclohexanone, butylacetate, propylacetate, gamma-butyrolactone, ethylhexanol, and ionic fluids, have been proposed to replace water. Commonly used non-polar solvents include alkanes such as decane, dodecane, and tetradecane, fluoroalkanes, aromatic and partially hydrogenated aromatic hydrocarbons, heterocyclic aromatic compounds, higher alcohols and carboxylic acids, and silicone oils.
Colored immiscible fluids are an indispensable part of electrofluidic and electrowetting devices, where reproduction of visual information and effects are required for the application. Conventional electrowetting devices typically have a colored oil that forms a film over an insulating fluoropolymer. This colored oil film imparts a visible color to the device. When a voltage is applied between a water layer situated above the oil film and an electrode beneath the insulating fluoropolymer, the oil film is disrupted as water electrowets the surface. The disrupted oil film no longer provides color to the device. Once the voltage is removed, the oil preferentially wets the insulating fluoropolymer, the oil film is reformed, and the color is again evident.
In general, the colorant can be a dye or a pigment. The dye is soluble in application media. That is, it is dispersed in the solvent to the molecular level or relatively small clusters (associates), for example dimers, trimers, tetramers and so on. The pigment is essentially insoluble in application media and exists in the form of particular dispersion. Historically, dyes have been the colorant of choice for various digital applications such as inkjet inks, color filters and electrowetting devices. This fact is not difficult to understand given the great variety of products demonstrating a range of vivid and intense colors, low viscosity of solution, excellent transparency and stability of colored fluids. Dyes however have certain disadvantages including poor light and weather fastness. Other disadvantages include high cost, especially for purified forms, inadequate solubility in non-polar solvents, low resistance to bleed, and/or a lack of opacity. In applications where dyes have been employed as coloring agents, organic pigments have been finding increased utility in recent years due to desirable light fastness and resistance to solvents and bleed. Pigments, on the other hand, are less transparent and stabilization of the pigment dispersion is always a serious concern.
While the problems associated with the use of water and other solvents, and colorants, are being addressed, there still remains a clear need for improved colored fluids for a variety of electrowetting, electrofluidic, and electrophoretic devices.
It would thus be beneficial to provide an improved colored fluid for electrowetting, electrofluidic, or electrophoretic devices that, for example, demonstrates minimal or no negative impact on device components, can enhance device performance, and maintain a desired function over a preferred period of time.